Image projector and method of operating same

ABSTRACT

The invention relates to an image projector with a High-Intensity-Discharge HID lamp ( 112 ) and a method of operating it. Such lamps have the disadvantage that their voltaic arc jumps back and forth between different positions at unforeseeable times during operation. This so-called arc jumping has the effect of changing the brightness, i.e. the overall brightness and/or the brightness distribution of the light emitted by the HID lamp ( 112 ). For the viewer of an image projected with such a projector, this effect appears as a jolt of this projected image. To avoid such effects for the viewer, it is first proposed according to the invention that changes in the brightness of the light in the image projector&#39;s beam path are detected. As soon as a change in brightness caused by an arc jump has been detected, this brightness is reset to a brightness detected at a moment t- 2  before the arc jump, in order subsequently to convert it during a predetermined time interval T to the brightness resulting from the arc jump. This resetting must occur so soon after the arc jump, and the conversion so slowly, that the changes in the brightness of the light incident on a picture screen device ( 118 ) resulting from the arc jump, the reset, and the conversion are not perceptible to the human eye.

This application is a 371 of PCT/IB02/05745 Dec. 18, 2002.

The invention relates to an image projector with aHigh-Intensity-Discharge HID lamp as defined in the preamble of claim 1and a method of operating it as defined in the preamble of claim 17.

Such image projectors with HID lamps are basically known from the priorart, where they are primarily used for video or presentation purposes.These lamps are distinguished by a very high luminous efficacy incombination with a small volume. However, they have the disadvantagethat their voltaic arc suddenly jumps in an uncontrolled way, i.e. atunforeseeable times. Jumping means that the arc takes up a new positionwhich is about 10 to more than 250 μm away from the original position.The human eye perceives this arc jumping as a brief jolt in a displayedor projected image. For people with sensitive eyes, this effect can bedisturbing when viewing an image, and it is therefore necessary toeliminate it.

The properties of the projection system connected to the output of thelamp imply that the arc jump has the physical effect of changing thebrightness, i.e. the brightness distribution and/or the overallbrightness, of the displayed image in an order of magnitude of a fewpercentage points.

EP 766906 A1 discloses a very effective method of avoiding the effect ofthe arc jump as such and the associated changes in the brightness of thelight in the image projector's beam path. The method disclosed thereinprovides for feeding the HID lamp with an electric current whose flowpresents an additional pulse shortly before commutation. However, adisadvantage of this known method is that it is not compatible withmodern display devices, which use a time-sequential display method. Suchdisplay devices require lamps with a constant overall brightness andbrightness distribution, which cannot be ensured with the method knownfrom EP 766906 A1 because of the additional current pulse.

Starting from this prior art, it is therefore the object of the presentinvention to develop a known image projector and a known method foroperating it, in such a way that the effects of the arc jump in the formof changes in the overall brightness and/or the brightness distributionof an image projected on a picture screen device are not perceptible tothe human eye.

This object is achieved by the subject matter of claim 1. Accordingly,the known image projector comprises a sensor device for detecting thebrightness, in particular the brightness distribution and/or the overallbrightness, of the light incident on the display field at differenttimes; a comparator device for comparing the brightness of the lightdetected by the sensor device at a previous moment t-2 and a moment t-1,which is later in time than moment t-2, and for generating a brightnesscontrol signal which represents a change in brightness occasioned by anarc jump that occurred in the HID lamp between the moment t-2 and t-1;and a control device for compensating the detected change in brightnessin response to the brightness control signal by resetting the brightnessof the light incident on the picture screen device (118) at a moment t0later than t-1, in particular in response to the brightness recorded atthe previous moment t-2, and by subsequent conversion of the resetbrightness during a predetermined time interval T to the brightnessrecorded at the later moment t-1, the resetting of the brightnessoccurring so soon after the arc jump, and the conversion of thebrightness during the time interval T occurring so slowly, that thechanges in the brightness of the light incident on the picture screendevice (118) caused by the arc jump, the reset, and the conversion arenot perceptible to the human eye.

For the present invention, the term “brightness” should always beunderstood as overall brightness and/or brightness distribution unlessstated to the contrary.

The brightness, in the sense defined above, of the light incident on thepicture screen device or of the image projected on the picture screendevice is influenced by the various components in the image projector'soptical system: As is known, the starting point for the light is the HIDlamp, which does not always couple the light into the optical system inthe same way, but varying in time especially because of the arc jump.This coupling at variable times results in a change in brightness of thelight emitted from the HID lamp in the course of time. Furthermore, boththe display field provided in the beam path and also an optical filterthat may be provided for special embodiments of the present inventionwill influence the brightness of the light before it hits the picturescreen device. Not the overall brightness, but the brightnessdistribution of the light will be influenced where applicable by anoptical integrator additionally provided in the beam path. For thepresent invention, the ideal case is assumed for the lens unit that itexerts no influence on the brightness of the light, i.e. it is regardedas ideally transparent. Since the invention relates to time-variantlight influences, there is no need at this point for a discussion ofother, stationary influences, such as those exerted by the lens unit.

Because of the many optical components in the beam path of the opticalsystem mentioned above, both the overall brightness and the brightnessdistribution of the light are different at different places in the beampath. As a result of the claimed embodiment of the image projector,disturbances or changes in the overall brightness and the brightnessdistribution of the light emitted from the HID lamp, caused inparticular by arc jumping in the HID lamp, have no effects visible tothe human eye in the light incident on the picture screen device or inthe image projected on this. This is achieved, as claimed in thedisclosure of claim 1, in that the described disturbances in thebrightness of the light output by the HID lamp are compensated for inthe beam path before the light hits the picture screen device. Thecompensation occurs with the resetting of the brightness at moment t0 tothe brightness of moment t-2 and the subsequent conversion of thebrightness during the period of time T to the brightness of moment t-1,as described in claim 1.

The claimed time ratios are especially important here. According to theinvention, the duration between an arc jump occurring, recognition ofthe change in the brightness by the comparison, and resetting of thebrightness of the light incident on the picture screen device to thebrightness at moment t-2 before the arc jump is chosen to be so shortthat the human eye does not perceive the change in brightness that hastaken place in the meantime; in other words, the human eye experiencesthe brightness at moment t0 as unchanged from that at moment t-2.

Only after the reset of the brightness at moment t0 is there a gradualadjustment/conversion to the changed brightness caused by the arc jump,according to the invention. This conversion of brightness to thebrightness detected at the later moment t-1 occurs so slowly that it islikewise not perceived by the human eye.

According to a first embodiment of the invention, the optical systemcomprises an optical integrator between the HID lamp and the displayfield, this integrator at least partially approximating a desireduniformity, not of the overall brightness but of the brightnessdistribution of the light incident on the picture screen device. Thepossibilities described in the following embodiments, for furthercompensation of the brightness distribution towards achieving to auniform distribution, are therefore only necessary if the compensationachieved by the integrator is not regarded as sufficient and additionalfinancial resources are available for the implementation of theembodiments described in the following, or the following embodimentswith the simpler integrator prove to be more favorable in terms of costor volume/weight.

According to a further embodiment, the resetting and conversion of thebrightness distribution and/or the overall brightness of the light inthe beam path, as claimed in the invention, is effected by anelectrically controllable optical filter arranged between the HID lampand the display field, or between the display field and the lens unit.Alternatively, the resetting and conversion may be implemented withoutthe optical filter by overlaying the images provided by the imageprocessor with a gray tone mask whose overall brightness and/orbrightness distribution is suitably set, before said images aredisplayed on the screen. In both cases, i.e. both when the opticalfilter is used and when the image processor is used, the resetting andconversion of the brightness occur in response to a brightness controlsignal provided by a comparator device.

Besides the described alternative usage of optical filter and brightnesscompensation in the image processor, a combination of both embodimentsis obviously also possible, one part of the necessary brightnesscompensation then being provided by the optical filter and another partby the image processor. Thus it is feasible in particular that theoptical filter only compensates the brightness distribution, while theimage processor regulates the overall brightness, or vice versa.Alternatively, however, it is also feasible that both the optical filterand the image processor regulate the overall brightness and thebrightness distribution in proportionate ratios.

It should be stressed again here that the optical filter and the imageprocessor—in contrast to the optical integrator—may implement abrightness compensation, i.e. the overall brightness and/or thebrightness distribution, of the light in the beam path; however, thisapplies only in the direction of lower brightness (loss of brightness).Thus a compensation by the optical filter and the image processorfilters out light, whereas a compensation by an integratorre-distributes the available light.

According to an advantageous development of the invention, thecompensation of the overall brightness, especially in the area of thebrightness reserve, may be implemented, alternatively or additionally tothe compensation by the image processor or by the optical filter, by acorresponding activation of the lamp driver. This has the advantage thatthen, if applicable, a compensation is even possible in excess of thefictitious nominal overall brightness of 100% that represents the imageprocessor's performance limit. Such a compensation above 100% may benecessary, for example, if the arc in the HID lamp jumps to positionsthat cause a reduction of the brightness greater than the brightnessreserve, or if no brightness reserve is to be provided (greater nominallight quantity). It may also be necessary in order to compensate for areduction in the HID lamp's emitted light quantity caused by an arc jumpafter resetting.

However, such a compensation of the light quantity is also advisable inthe opposite case, i.e. if an increase in the brightness of the lightcaused by the arc jumping must be compensated for by a brief reset ofbrightness at moment t0, not to the brightness at moment t-2, but to aslightly reduced light quantity in comparison. In both cases, anequalization of the light quantity has the advantage that theresetcontrol can take place more slowly without visible faults arisingin the image.

An electronic circuit for regulating the overall brightness of the lightin an image projector's optical system through the lamp control unit isknown, for example, from the not previously published German patentapplication with application number 10136474.1.

In contrast to the compensation by means of the image processor and/orthe optical filter, a compensation of the brightness distribution by thelamp driver and the lamp control unit is not possible.

To detect the overall brightness, it is basically sufficient if thesensor device comprises only one sensor element. On the other hand, fordetecting the brightness distribution of the light incident on thedisplay field, at least two sensor elements positioned in differentplaces are needed.

The above object of the present invention is further achieved with amethod as claimed in claim 17 for operating the described imageprojector. The advantages of this method correspond to the advantagescited above for the image projector. According to an advantageousembodiment of the invention, the method provides that the brightnesscompensation necessary for a particular arc jump is calculated by meansof a mathematical compensation time function.

Further advantageous embodiments of the claimed projector and theclaimed method are the subject matter of the dependent claims.

The invention will be further described with reference to embodimentsshown in the drawings to which, however, the invention is notrestricted.

FIG. 1 shows the hardware construction of an image projector accordingto a first embodiment of the invention;

FIG. 2 a shows the change of brightness of the light incident on thepicture screen device over time, according to the invention;

FIG. 2 b shows a modification of the method according to the inventionwith light quantity compensation;

FIG. 2 c shows the relationship between the brightness of the lightemitted by the HID lamp and the brightness of the light incident on thepicture screen device for a reduction in brightness caused by an arcjump;

FIG. 2 d shows the relationship between the brightness of the lightemitted by the HID lamp and the brightness of the light incident on thepicture screen device for an increase in brightness caused by an arcjump;

FIG. 3 shows a second, third, and fourth embodiment of the hardwareconstruction of the image projector according to the invention;

FIG. 4 shows a fifth embodiment of the hardware construction of theimage projector according to the invention;

FIGS. 5 a and b show the change in brightness distribution before andafter the arc jump in a general case;

FIGS. 5 c and d show the change in brightness distribution which isuneven in one direction only, before and after the arc jump;

FIG. 5 e shows a brightness distribution for which only one sensor isneeded; and

FIG. 6 shows the estimate of the shift of the maximum of the brightnessdistribution because of an arc jump.

FIG. 1 shows a first embodiment of the image projector 100 according tothe invention. This comprises an optical system 110 for displaying asupplied image on a picture screen device 118. The optical system 110comprises a High-Intensity-Discharge HID lamp 112, in particular anUltra High Pressure UHP lamp, and arranged after this an opticalintegrator 112 a, a display field 114, and a lens unit 116 forprojecting the supplied image onto the picture screen device 118. Theimage to be projected is made available to the display field 114 by animage processor 120.

The optical integrator 112 a serves to compensate at least partially foruneven distributions of brightness in the light emitted by the lamp 112,with a view to achieving uniform distribution. The image projector 100also comprises a lamp driver 131 for supplying a suitable operatingvoltage to the HID lamp 112. The lamp driver 131 itself is fed from anexternal voltage U.

According to the invention, the brightness, i.e. the brightnessdistribution and/or the overall brightness, of the light emitted fromthe HID lamp 112 and incident on the display field 114 is detected by asensor device 140 at different moments. The brightnesses thus detectedare fed to a comparator device 150, in order to detect any changes inthe brightness in the course of time. The comparator device 150generates a brightness control signal which represents changes detectedin the brightness in the course of time by a comparison in particular.The brightness control signal is fed to a control device, i.e. thecomponent 120′ of the image processor 120. This component 120′ isdesigned such that it adjusts the brightness of the image output by theimage processor 120 to the display field 114 in response to thebrightness control signal as in a method described below according tothe invention such that a change in the light's brightness caused by anarc jump in the HID lamp 112 is not perceived by a viewer of the imageon the picture screen device 118. In other words, the component 120′ ofthe image processor 120 compensates for changes arising in thebrightness in response to the brightness control signal.

FIG. 2 a illustrates the method according to the invention forcompensating a change of brightness of the light in the beam path of theoptical system 110, caused by an arc jump in the HID lamp 112. A changeof brightness, i.e. of brightness distribution and/or overall brightnessof the light, may arise as a result of the arc jump. FIG. 2 aillustrates the method in the first place with the example of a changeof the overall brightness of the light incident on the picture screendevice 118 in the negative direction, i.e. for the case of a reductionin the overall brightness because of the arc jump. Secondly, FIG. 2 aillustrates the method according to the invention for compensating achange in the brightness distribution caused by the same arc jump,different curve characteristics then arising for each picture element.

As can be seen from FIG. 2 a, the overall brightness is initially takenas constant before an arc jump. This constant brightness is detected bythe sensor device 140 at the moment t-2. If the same sensor device 140detects a reduced brightness at a later moment t-1 compared with themoment t-2, this reduction in the overall brightness will be recognizedby the comparator device 150.

The method according to the invention provides that immediately after achange of brightness has been registered in the comparison, this changeis reversed by resetting of the brightness. It is important that thereset to the brightness present before the arc jump at moment t-2 shouldoccur within a few milliseconds after the arc jump; this is the only wayto ensure that the viewer of the image projected onto the picture screendevice 118 does not register the change of brightness caused by the arcjump by seeing a jolt in the image. The method according to theinvention provides that after the reset—as illustrated in FIG. 2 a—, thebrightness reset to the value before the arc jump is slowly converted tothe brightness resulting from the arc jump. It is important here thatthis conversion should occur so slowly that the human eye observing theimage on the picture screen device 118 also fails to notice thisconversion of the brightness as a fluctuation in the projected image.The conversion is therefore advantageously executed over a time intervalT which is made sufficiently long. The length of the time interval T isusually a few seconds, adjusted for the light/dark adaptation of theviewer's eye; but in any case it is considerably longer than the elapsedtime between the arc jump and the reset of the brightness at moment t0.The inertia of the human eye is utilized in both the fast resetaccording to the invention and also the slow conversion of thebrightness.

As is also shown in FIG. 2 a, an analogous compensation of a changedbrightness distribution caused by the arc jump (distribution 1 todistribution 2) initially occurs with a reset to the distribution 1 anda subsequent conversion of the distribution 1 to the distribution 2.Examples of the distribution 1 before the arc jump are illustrated inFIGS. 5 a+c; the respective associated distributions 2 after the arcjump are shown in FIGS. 5 b+d.

FIG. 2 b shows a modification of the method according to the inventionas illustrated in FIG. 2 a. It is aimed at compensating the lower lightquantity emitted in the period between an arc jump occurring and thesubsequent resetting of the brightness of the HID lamp with respect toan unchanged arc position represented by the area F1. According to theinvention, the compensation comprises a brief overloading for a few msof the lamp 112 after the moment t0. Ideally the lamp 112 in this caseemits exactly the additional light quantity represented by the area F2,which is equal to the reduction in light which previously reached thedisplay field in the interval between the arc jump and to; i.e. F2 ispreferably equal to F1. As can be seen from FIG. 2 b, the duration ofthe compensation is much shorter than the duration of the time intervalT.

Since the additional light quantity F2 needed for the compensation isneeded for a very short time only, it is easily possible to implementthis with a brief overloading of the HID lamp 112. Accordingly, a lampcontrol unit 132 assigned to the lamp driver 131 is preferably designedsuch that in this case it briefly, i.e. for a few seconds only,overloads the HID lamp 112. Such brief overloads have no negative effecton the overall life of the HID lamp 112, especially as thecharacteristics of the arc jumps mean that underloads occur just asoften.

The compensation of the light quantity just described for the case of areduction in brightness caused by the arc jump may occur equally well incase of an increase in brightness caused by the arc jump. Thecompensation then occurs after the moment t0 in the form of a brieflyreduced light quantity emitted by the HID lamp 112.

As stated above, FIG. 2 a shows the change of brightness of the lightincident on the picture screen device 118. However, this brightness isnot identical to the brightness of the total light output from the HIDlamp 112; this is hardly changed by the arc jump.

FIG. 2 c schematically illustrates the relationships between thebrightness of the light emitted by the HID lamp 112 and the brightnessof the light incident on the picture screen device 118 for a reductionof brightness caused by an arc jump. As can be seen from FIG. 2 c, thebrightness of the light incident on the picture screen device 118 as inFIG. 2 ciii) at any moment is the result of a compensation of thebrightness of the light emitted by the HID lamp 112 to the display field(114) as in FIG. 2 ci) via the various optical system elementspost-connected to the HID lamp 112, such as especially the opticalintegrator 112 a and the display field 114; see FIG. 2 cii).

According to an advantageous embodiment of the method described abovewith reference to FIG. 2 a according to the invention, it is especiallyadvantageous for a compensation of the overall brightness by the imageprocessor component 120′ to provide a brightness reserve, i.e. aso-called headroom, as indicated in FIG. 2 cii). This means that theimage is not output to the display field 114 with a maximum nominaloverall brightness of 100% for the image processor, but with a reducedoverall brightness compared to this. The image processor's brightnessreserve can be used to compensate a reduction in overall brightnesscaused by an arc jump. The compensation advantageously occurs so fastthat the reduction in the overall brightness resulting from the arc jumpis not perceived by the human eye. However, the described embodiment hasthe disadvantage that—even at a time when no brightness compensation isnecessary because no arc jumping occurs—the nominal overall brightnessof 100% for the display of the image on the picture screen device, astheoretically possible for the HID lamp and the image processor, is notfully utilized, this image instead being displayed with a reducedoverall brightness only.

In order to at least partially eliminate this disadvantage, use of afurther property of arc jumping is proposed. Because of the technicalproperties of the HID lamp, the arc jumps typically occur in phases,i.e. in several hours of operation there is no arc jumping at all, oronly very occasionally. In contrast, during other phases, arc jumpingoccurs frequently. Thus, by evaluating the signals from the sensordevice 140, the comparator device 150 can detect, for example, whether aphase with or without arc jumping is present. If no further arc jumpinghas occurred for at least a predefined time interval Δt1, for example 10minutes, the overall brightness is slowly run up to 100%. This 100%overall brightness is maintained until the arc jumping resumes. Forexample, the comparator device may then calculate how much brightnessreserve should be allowed again for a full compensation of the currentlystrongest arc jumps; an algorithm used for the calculation thentypically specifies this newly required brightness reserve in the formof greater than 100% transmittance. The basic brightness is then reducedonce more, i.e. the brightness reserve is then built up again, until thegreatest change in brightness detected by the algorithm and caused bythe arc jumping can once more be compensated with 100% transmittance ofthe image output to the display field 114. The corresponding value forthe overall brightness is then maintained until the comparator deviceagain detects that no further arc jumping has occurred for at least thetime interval Δt1, for example 10 minutes.

The result of this control mechanism is in fact that the first jumpresult cannot be compensated and is therefore fully visible. But theeffects of the subsequent arc jumps on the overall brightness of thelight are increasingly reduced within a few seconds, until after a whilethe image appears completely stable again, in spite of ongoing arcjumping. During the time in which the arc jumping occurs, the brightnessis then reduced by a maximum changed brightness value resulting from abriefly occurring arc jump, i.e. the brightness reserve is increased; inlong, quieter phases the brightness rises to 100% again.

Both the reduction of the brightness reserve during long absences of arcjumping and the increase in the brightness reserve when arc jumpingsubsequently resumes occur so slowly that the accompanying changes ofthe overall brightness cannot be perceived by the human eye because ofits inertia.

This variable adaptation, i.e. the sliding brightness reserve, can bequite easily implemented by charging a capacitor through a suitablyselected resistance to a voltage value that represents the amplitude ofthe current arc jump, and subsequently discharging the capacitor througha suitable larger resistance. The voltage across the capacitor is thenproportional to the necessary brightness reserve. The values of theresistances determine the desired time constants.

The provision of the described brightness reserve for the compensationof the overall brightness is not restricted to the case where theoverall brightness is compensated by the image processor. The brightnessreserve may be similarly provided if the compensation of the overallbrightness is effected by the optical filter.

FIG. 2 d schematically illustrates—as a virtual counterpart to FIG. 2c—the relationships between the brightness of the light emitted by theHID lamp 112 and the brightness of the light incident on the picturescreen device 118 in the case of an increase in the brightness of thelight emitted by the HID lamp 112 because of an arc jump. As can be seenfrom FIG. 2 d, the compensation occurs precisely inversely to the casein FIG. 2 c, so no further explanation of FIG. 2 d is given.

For the implementation of the method just set forth according to theinvention for compensation of a change, i.e. reduction or increase inthe brightness of the light incident on the picture screen device 118caused by an arc jump, a number of embodiments are proposed according tothe invention for the image projector 100; these are described in moredetail below, again with reference to FIG. 1, but also with reference toFIGS. 3 to 6.

In the above embodiment shown in FIG. 1, the brightness compensationaccording to the invention is exclusively effected by the imageprocessor 120. The brightness control signal informs the component 120′of the image processor 120 of a brightness change that occurred as aresult of an arc jump, i.e. a change in the overall brightness and/orthe brightness distribution, for the light incident on the display field114. In response to this brightness control signal, the component 120′then sets the brightness (transmittance) of the image output to thedisplay field 114 in such a way that the brightness of the lightincident on the picture screen device 118 is unchanged with respect tothe time before the arc jump. For the example of the overall brightnessof the light emitted by the HID lamp being reduced by an arc jump—asshown in FIG. 2 ci)—the component 120′ as a control device will first(i.e. at moment t0) increase the brightness (transmittance) of the imageoutput to the display field 114 in accordance with the amount by whichthe brightness has been reduced. More precisely, the component 120′ willincrease the light transmission of the image to the extent that—as shownin FIG. 2 ciii)—at moment t0 the brightness of the light incident on thepicture screen device 118 is once again unchanged compared with the timebefore the arc jump. Very gradually during the subsequent conversionphase, i.e. during a predetermined time interval T, the component 120′will then once again reduce the brightness or transmittance of the imageto the extent that upon expiry of this time interval T the brightness ofthe light incident on the picture screen device 118 is adjusted to thechanged brightness resulting from the arc jump. The reset and theconversion are preferably executed with a predetermined compensationtime function which is independent of a concrete embodiment.

FIG. 3 shows a second, third, and fourth embodiment for implementing themethod according to the invention, described above with reference toFIG. 2. According to the second embodiment, the compensation, i.e. theresetting and the conversion of the brightness, is not executed withimage processors but with an optical filter 113 arranged in the beampath in front of or behind the display field 114, this optical filterrepresenting the control device in this case. This optical filter 113can be controlled electronically in response to the brightness controlsignal output by the comparator device 150. While the image processor120 in this second embodiment simply outputs the image for projectionwith a constant brightness to the display field 114, the compensationaccording to the invention for the light in the beam path follows fromsaid activation of the optical filter by the brightness control signal.The optical filter 113 is preferably designed as a gray tone mask, sothat the overall brightness and/or the brightness distribution of thelight in the beam path can be influenced as necessary by appropriateactivation. In other words, in the method according to the invention,the optical filter 113 in the second embodiment, just like the component120′ of the image processor 120 in the first embodiment, enables acompensation for the change caused by an arc jump in the brightness ofthe light incident on the picture screen device 118.

The third embodiment is likewise shown in FIG. 3. It is a combination ofthe first and second embodiments. More precisely, it provides for aimplementation of the compensation according to the invention for thebrightness of the light, according to a predetermined proportionateratio, by simultaneous use of both the component 120′ and the opticalfilter 113; in this case, they together form the control device. Forthis purpose it is necessary for the brightness control signal to befed, not only to the optical filter 113, but also—as indicated by thedotted line in FIG. 3—to the component 120′. The pending compensationtask may then be apportioned as desired to the optical filter 113 andthe component 120′. Thus it is feasible, for example, that the component120′ only performs any necessary compensation of the brightnessdistribution, while the optical filter 113 implements any necessarycompensation of the overall brightness. The opposite case is obviouslyalso feasible. As an alternative to these two variants, the component120′ and the optical filter 113 may also each be used for compensatingboth the overall brightness and the brightness distribution; but adefinition of the respective percentage shares of the component 120′ andthe optical filter 113 in the compensation of the brightness is thennecessary.

If the optical filter 113 is not activated, the third embodimentcorresponds to the first, and if the component 120′ of the imageprocessor is not activated, the third embodiment corresponds to thesecond embodiment.

It holds for all three embodiments of the image projector described sofar that the use of the optical integrator 112 a in the optical system110 only necessitates a compensation of the brightness distribution bythe optical filter 113 and/or the component 120′ of the image processor120 to the extent that a compensation of the brightness distributionover and above the compensation achieved by the optical integrator 112 ais required. In other words, if the compensation of the brightnessdistribution achieved by the optical integrator 112 a is judged to beadequate, the optical filter 113 and/or the component 120′ only have toimplement a compensation of the overall brightness. This would be thefourth embodiment shown in FIG. 3 for the image projector 100. Anadequately compensated brightness distribution is shown in FIG. 5 ebelow and described by way of example in the associated description.

FIG. 4 shows a fifth embodiment for the hardware construction of theimage projector 100 according to the invention. The hardwareconstruction for this fifth embodiment differs from the hardwareconstruction of the third or fourth embodiment as in FIG. 3 only in thatthe comparator device 150 generates not only the brightness controlsignal for the optical filter 113 and/or the component 120′ of the imageprocessor 120, but also a light quantity signal for the lamp controlunit 132. Here the lamp control unit 132, together with the opticalfilter 113 and/or the component 120′ where applicable, forms the controldevice.

The light quantity signal represents the change in the overallbrightness caused by an arc jump, in so far as this is not to becompensated by the component 120′ of the image processor 120 or theoptical filter 113. In response to this light quantity signal, the lampcontrol unit 132 therefore enables, either alone or in addition to theoptical filter 113 and/or the component 120′, a compensation of thechange in the overall brightness caused by an arc jump. In contrast tothe optical filter 113 and the component 120′, the lamp control unit 132implements the compensation of the overall brightness by a correspondingactivation of the lamp 112. In other words, depending on the necessarycompensation direction, the lamp control unit 132 feeds more or lesspower to the HID lamp 112, so that the lamp 112 emits a brighter ordimmer light. In contrast to the optical filter 113 and the component120′, the lamp control unit 132 is only able to compensate the overallbrightness, and not the brightness distribution. But in relation to theoverall brightness, the necessary compensation performance can beapportioned as desired among the lamp control unit 132, the opticalfilter 113, and the component 120′ of the image processor 120 (asdescribed above in relation to the filter and the component 120′ for thethird embodiment).

Any necessary compensation of the brightness distribution must also beperformed by the optical filter 113 and/or the component 120′ inresponse to the brightness control signal in the fifth embodiment.

For all described embodiments, the regulation in particular of theoverall brightness as in the method according to the invention may alsoinclude the provision of a brightness reserve, i.e. a so-calledheadroom, as described above with reference to FIG. 2 c.

As described above in the description of the method according to theinvention with reference to FIG. 2 a, the compensation of thebrightness, i.e. the resetting and conversion of the brightness, isexecuted in accordance with a compensation time function.

FIGS. 5 a to 5 e and 6 illustrate the calculation of this filterfunction with reference to the example of some typical brightnessdistributions with a more or less ideal integrator 112 a.

FIG. 5 a shows schematically a brightness, in particular a brightnessdistribution before an arc jump, for example at the moment t-2. Thedistribution function indicated here is uneven in both directions (X andY), and therefore requires at least 3 sensors. FIG. 5 a shows an exampleof the possible arrangement of 4 sensor elements 140-1 to 140-4 on theedge of the reference plane 6. In contrast, FIG. 5 b shows a brightnessdistribution after an arc jump, for example at moment t-1. Thedifferences in the spherical deformation of the two curved surfaces inFIG. 5 a and FIG. 5 b represent the differences in the brightnessdistribution caused by the arc jump. In contrast, the differences inheight of the two curved surfaces in FIG. 5 a and FIG. 5 b compared withthe reference plane 6 for one equal point in each case, i.e. one equalgeometric location in the reference plane 6, represent the differencesin the overall brightness for this point before and after the arc jump.The reference plane 6 is at an angle to the beam path, it isrepresented, for example, by the plane of the display field 114.

FIG. 5 c illustrates the special case that the brightness distributionis uneven in one direction only (here the X direction); such directionsin which the brightness distribution is uneven are known in some opticalsystems 110. In such a case, only two sensor elements 140-1 and 140-2are needed. FIG. 5 d shows such a brightness distribution after an arcjump.

FIG. 5 e shows an almost even brightness distribution over the referenceplane 6 as achieved by a good integrator 112 a. The deviations in thedistribution of this from an ideal uniform distribution are so minorthat a shift in the distribution—caused by arc jumping—is not seen bythe viewer as a disturbance. In this case there is no need to record andevaluate the shift in brightness distribution; one sensor element 104-1is then sufficient to measure and correct the overall brightness.

The evaluation of the detected change in brightness distribution for thepurpose of calculating the compensation time function will now bedescribed in more detail with reference to FIG. 6. In the first place,it can be seen from FIG. 6 that the sensor device 140 consists, forexample, of four sensor elements 140-1 . . . 140-4 which are arrangedcentrally on the edges of the rectangular display field 114, representedby the reference plane 6. Alternatively, the sensor elements 140-1 . . .140-4 of the sensor device 140 may be arranged on the corners of thedisplay field 114. In principle, however, other sensor arrangements, forexample as presented above in the description of FIGS. 5 a-d, are alsopossible depending on the individual case; the correspondingcalculations may easily be adapted by the expert.

The sensor elements 140-n measure the brightness at their respectivepositions at the previous moment t-2. When the brightness values thusdetermined are viewed as a whole, the brightness distribution over thedisplay field 114 at moment t-2 can be estimated. The same procedure isthen repeated for determining the brightness distribution at the latermoment t-1. The situation—dependent on the current arc position—of thebrightness distribution and its amplitude (=overall brightness) at anytime can be uniquely described, for example by a vector, from theknowledge of the brightness distribution function, i.e. the mathematicalrelationship of the brightness distribution over the area of the displayfield 114 of the optical system 110. If a difference is then detected ina subsequent comparison between the brightness distribution recorded atthe later moment t-1 and that recorded at the previous moment t-2, thisdifference is represented by a difference vector(vector_(t-1)−vector_(t-2))), which, for example, as shown in FIG. 6,represents a shift of the brightness maximum at moment t-1 compared withmoment t-2. The length of the vertical component of this differencevector is then a measure for the change in the overall brightness of thelight incident on the display field 114. From this vector a prototypefunction is calculated which mathematically describes the change in thebrightness distribution over the entire display field 114. The inversefunction of this prototype function is the said compensation timefunction which, as described above, defines the necessary compensationof the brightness of the light incident on the picture screen device 118over time.

For resetting the brightness at moment t0, as described above withreference to FIG. 2 a–d, the compensation time function—as illustratedin FIG. 2 c) ii)—represents precisely the difference between the twobrightnesses measured at the later moment t-1 and the previous momentt-2. From the moment t0, this compensation time function is thenincreasingly damped during the predefined time interval T until atmoment t0+T it has finally faded to zero. At the moment t0+T,compensation is no longer needed for the brightness, because then thebrightness as originally caused by the arc jumping is now set, i.e. onthe picture screen device 118.

In general, the more sensor elements are used, the more precisely theamount of deviation of brightness distribution from a uniformdistribution can be measured in the other areas. But the more preciselyalso can a time change or shift of the brightness distribution betweenthe later moment t-1 and the previous moment t-2 be recorded, and themore precisely too can the compensation time function be calculated,which then in turn enables a more precise compensation of the change inbrightness.

In the fourth embodiment, in which a brightness according to FIG. 5 e isassumed, one sensor element is sufficient for recording the overallbrightness, as explained above in the description for FIG. 5 e. There isthen no need for the calculation described above with reference to FIG.6 for the brightness distribution and changes to it with the help of theprototype function and the compensation time function.

Altogether, the combination according to the invention of compensationof the brightness distribution with compensation of the overallbrightness, as described above in several embodiments, achieves a stableoperating state with a subjectively constant brightness for the viewerof the image projected on the picture screen device 118. As a result ofthis double regulation of brightness distribution and overallbrightness, an arc jump that has actually occurred remains unnoticed bythe viewer; in particular, he will advantageously notice no jolt in theprojected image in spite of the arc jump that has occurred.

1. An image projector (100) with: an optical system (110), comprising aHigh-Intensity-Discharge HID lamp (112), in particular an Ultra HighPressure UHP lamp, and arranged after this a display field (114) and alens unit (116) for projecting an image prepared on the display field(114) through the lens unit (116) onto a picture screen device (118); alamp driver (131) for providing a supply voltage to the HID lamp; and animage processor (120) for providing the image for the display field(114); characterized by a sensor device (140) for detecting thebrightness, in particular the brightness distribution and/or the overallbrightness, of the light incident on the display field (114) at varioustimes; a comparator device (150) for comparing the brightness of thelight detected by the sensor device (140) at a previous moment t-2 and amoment t-1, which is later in time than moment t-2, and for generating abrightness control signal which represents a change in brightness causedby an arc jump that occurred in the HID lamp (112) between the momentst-2 and t-1; and a control device (113, 120′, 132) for compensating thedetected change in brightness in response to the brightness controlsignal by resetting the brightness of the light incident on the picturescreen device (118) at a moment t0 later than t-1, in particular to thebrightness detected at the previous moment t-2, and by subsequentconversion of the reset brightness during a predetermined time intervalT to the brightness detected at the later moment t-1, the resetting ofthe brightness occurring so soon after the arc jump, and the conversionof the brightness during the time interval T occurring so slowly thatthe changes in the brightness of the light incident on the picturescreen device (118) caused by the arc jump, the reset, and theconversion are not perceptible to the human eye.
 2. An image projector(100) as claimed in claim 1, characterized in that the optical system(110) comprises an optical integrator (112 a) connected downstream ofthe HID lamp (112) for an at least partial compensation of thebrightness distribution of the light incident on the display field (114)so as to achieve a uniform distribution.
 3. An image projector (100) asclaimed in claim 1, characterized in that the control device is designedas an electrically controllable optical filter (113) and is arranged inthe beam path of the optical system (110) in front of or behind thedisplay field (114).
 4. An image projector (100) as claimed in claim 3,characterized in that the optical filter (113) is designed as anadjustable gray tone mask.
 5. An image projector (100) as claimed inclaim 1, characterized in that the control device—if applicable, inaddition to the optical filter (113)—comprises a component (120′) of theimage processor (120) for resetting and converting the brightness of theimage provided by the image processor (120) for the display field (114)in response to the brightness control signal.
 6. An image projector(100) as claimed in claim 5, characterized in that the image processor(120) is designed to output the image—so long as no arc jumpingoccurs—to the display field (114) with an overall brightness preferablyreduced by a few percentage points compared with a nominal overallbrightness of 100%; in order that, when a reduced overall brightness ofthe light compared with the moment t-2 has been detected in thecomparison with moment t-1 because of arc jumping that took place in themeantime, it can raise the overall brightness of the image generated bythe image processor (120) for the display field (114) to a maximum of atmost 100% of the overall brightness in response to a part of thebrightness control signal relating to the overall brightness so fastthat this change in the overall brightness is not perceptible to thehuman eye.
 7. An image projector (100) as claimed in claim 5,characterized in that the image processor (120) is designed—if nofurther arc jumping has occurred for at least a predetermined timeinterval Δt1—to increase the overall brightness of the image output tothe display field (114), starting from a reduced overall brightness, toa nominal overall brightness of 100%, in order subsequently—if the arcjumping has resumed—once again to reduce the basic brightness of theimage to be output to the display field (114) to a suitable overallbrightness value to compensate for the change in the overall brightnesscaused by the arc jumping.
 8. An image projector as claimed in claim 7,characterized in that both the raising of the overall brightness to 100%and also its reduction occur so slowly, preferably over a few seconds,that they are not perceptible to the human eye.
 9. An image projector(100) as claimed in claim 1, characterized in that the control device,preferably in addition to the optical filter (113) and/or the component(120′) of the image processor, comprises a lamp control unit (132)connected to the lamp driver (131) for controlling the light quantityemitted by the HID lamp (112) into the optical system (110) in responseto a light quantity signal generated by the comparator device (150),which signal represents the overall brightness component of thebrightness control signal, in so far as the overall brightness is notalready being compensated by an appropriate activation of the opticalfilter (113) or the image processor (120).
 10. An image projector (100)as claimed in claim 9, characterized in that the lamp control unit (132)is designed to implement a necessary compensation in excess of thenominal overall brightness of 100%, in compensation for the reduction inthe overall brightness caused by the arc jumping.
 11. An image projector(100) as claimed in claim 10, characterized in that the lamp controlunit (132) is designed to activate the HID lamp (112) such thatimmediately after the moment to the lamp (112) additionally feeds thesame light quantity (F2) into the optical system as it fed too little(F1), in relation to a nominal light quantity, into the optical systemin the period between the previous arc jump and the moment t0 owing to areduction in the brightness of the light caused by the arc jumping; orthe lamp control unit (132) is designed to activate the HID lamp (112)such that immediately after the moment t0 the lamp (112) feeds the samelight quantity (F2) less into the optical system as it fed too much(F1), in relation to a nominal light quantity, into the optical systemin the period between the previous arc jump and the moment t0 owing toan increase in the brightness of the light caused by the arc jumping.12. An image projector (100) as claimed in claim 1, characterized inthat the sensor device (140) is designed for simultaneous detecting ofthe overall brightness and/or the brightness distribution of the light.13. An image projector (100) as claimed in claim 12, characterized inthat the optical system (110) comprises an optical light decouplingdevice, in particular a dichroic passive reflector, for decoupling arepresentative portion of the light incident on the display field (114);and that the sensor device (140) is arranged in a suitable positionoutside the beam path for detecting the decoupled part of the light. 14.An image projector (100) as claimed in claim 1, characterized in thatthe sensor device (140) comprises at least two sensor elements (140-1,140-2, . . . ) arranged in different locations in the beam path of theoptical system.
 15. An image projector (100) as claimed in claim 14,characterized in that the sensor elements (140-1, 140-2, . . . ) arearranged in a distributed manner over the surface and/or on the edge ofthe display field (114).
 16. An image projector (100) as claimed inclaim 15, characterized in that the display field (114) is polygonal, inparticular rectangular in design, and that a sensor element is arrangedcentrally on an edge portion and/or on the corners of the display field(114) each time.
 17. A method of operating an image projector (100) witha High-Intensity-Discharge HID lamp (112), in particular an Ultra HighPressure UHP lamp, a display field (114), and a lens unit (116),comprising the step of: projecting an image prepared on the displayfield (114) with the help of the HID lamp (112) through the lens unit(116) onto a picture screen device (118); characterized by: detectingthe brightness, in particular the overall brightness and/or thebrightness distribution of the light incident on the display field (114)at various times; comparing the brightness detected for the light at aprevious moment t-2 and a later moment t-1, and if at the later momentt-1 a change of brightness caused by an arc jump that occurred in theHID lamp (112) was detected compared with the previous moment t-2,compensating the detected change in brightness of the light by resettingthe brightness of the light incident on the picture screen device (118)in particular to the brightness recorded at the previous moment t-2, inorder subsequently to convert this brightness during a predeterminedtime interval T to the brightness recorded at the later moment t-1, theresetting occurring so soon after the arc jumping, and the conversion ofthe brightness during the time interval T occurring so slowly, that therespective related changes in the brightness of the light incident onthe picture screen device (118) are not perceptible to the human eye.18. A method as claimed in claim 17, characterized in that it isestablished in the comparison of the brightnesses whether a geometricshift in the location of the maximum for the brightness distributionand/or a change in the overall brightness of the light detected by thesensor device (140) has occurred between the previous moment t-2 and thelater moment t-1.
 19. A method as claimed in claim 18, characterized inthat the step for resetting the brightness comprises the followingsub-steps: calculation of a mathematical compensation time functionwhich represents the detected change in the brightness; and modificationof the current brightness of the light incident on the picture screendevice (118) in accordance with the compensation time function such thatthe brightness of the light is once again set to that at the previousmoment t-2.
 20. A method as claimed in claim 17, characterized in thatthe step of converting the brightness of the previous moment t-2 to thatof the later moment t-1 occurs in accordance with an increasing dampingof the compensation time function over the time interval T in apredetermined way.
 21. A method as claimed in claim 17, characterized inthat the scale of the time interval T is in the range of a few seconds.